All posts are valid checks. One more to check is the brushes on the alternator. The brushes are integral to the voltage regulator. It is attached to the back of the alternator by two screws. It's about 3/4x2". Disconect the neg batt cable and remove the two screws and remove the regulator/brush pack. The brushes should protrude about 3/4" from the regulator body when new. If the length of the brushes is dramatically different or they are both extremely short, replace the regulator. They can be found at auto parts stores for 20-40 dollars. About 50 at the dealer.

/// Check the negative battery cable. YOu cannot see the joint inside the batttery cable end clamp. If this joint opens up due to reistance and heat the battery will not charge and the car may not run on the unfilted alternator output. Use both of your jumper cable clamps to bypass that cable. I have found that the insulator can fail at the alternator bolt so that the output is shorted to the alternator case by that insulator failure. This problem will not come and go once that insulator fails.

Failed ring diodes will lower the output of the alternator. Some test equipmant can detect missing pulses.

A sudden demand for current makes the alternator very hard to turn and if you have loose or worn belts the alternator may stop turning at that current demand. Test by attempting to turn the alternator by pushing on the alternator cooling fan fins on a stopped engine.

Check all wires to alternator, and be prepared to fix the wiring hanress if all these connections are good. My 83' turbo's wiring harnes failed me this last summer, and i had to take it apart and rebuilt it. All of my wires were thrashed, and almost all were missing a foot or more on insulation. I had the same symptoms as you, no charging, or intermettent charging of the battery.

Mark,

Check the ground wire from the alternator to the engine block.

Skip

'80 242GT ProRally

'93 850GLT